1. Field of the Invention
The invention relates in general to fluid-cooled electrical apparatus such as power transformers and in particular to an improved fluid circulating pump used in such apparatus.
2. Description of the Prior Art
Electrical power transformers are commonly cooled with an insulating and cooling dielectric fluid such as mineral oil. Those with higher KVA ratings require forced cooling in order to keep their size reasonable for production, shipping and installation. Forced cooling is accomplished by a pump or pumps pulling the oil from external radiator-type heat exchangers and forcing it through the transformer.
Generally, these oil circulation pumps have the motor portion in fluid communication with the pump portion of the oil pump. Alternate possible designs could have the motor sealed from the pump by a stuffing box surrounding a common shaft. Pumps with the motor portion sealed off from the pump portion have inherent cooling, lubrication and maintenance disadvantages. The motor must be cooled by some means. Air-cooled motors are much larger than their oil-cooled counterpart and require periodic lubrication. Oil-cooled motors that are sealed from the impeller side of the pumping unit require an expensive sealed oil-to-oil or oil-to-air heat exchanger for cooling. Also required are provisions to accommodate oil expansion with increasing temperature within the sealed system. Thus, pumps having oil-cooled motors that are sealed from the impeller side of the pumping unit are not practical from an economic viewpoint. Both types of alternate units require periodic maintenance of the means for sealing the common shaft.
Since a transformer is relatively maintenance-free and is generally unattended, the advantages of a pump unit with the motor portion in fluid communication with the impeller or pump portion can be appreciated. This design allows elimination of the shaft sealing means and its inherent maintenance. In addition, a small portion of the transformer oil being pumped through the apparatus is circulated through the motor for cooling and lubrication purposes, thus reducing the size and cost of the unit over the alternatives discussed above. However, wear of the metallic parts of pumps of this design causes contamination of the dielectric cooling fluid with submicron-sized electrically conductive particles which are then distributed throughout the transformer by the pump. This electrically conductive particle contamination tends to reduce the dielectric properties of the insulating and cooling fluid as well as any solid dielectric material within the transformer on which the particles might collect. In addition, contamination of gross amounts of nonelectrically conductive particles can also be damaging to the dielectric properties of the insulating fluid.
Non-metallic or non-electrically conductive bearings alone will not solve this contamination problem. Should a bearing fail, metallic particles would still be rubbed off the rotor and stator as these components cannot be replaced with non-electrically conductive substitutes. Also, an uncontrolled quantity of non-electrically conductive particles could be present from wear of non-electrically conductive bearings. This contamination problem of conductive and/or non-conductive particles is even present in the isolated motor/impeller design since, should a bearing completely fail, the shaft seal would likely be damaged, allowing contaminants on the motor side to migrate to the impeller side and be circulated throughout the system. The only previous known arrangement that would not be plagued with this contamination problem would be a separate motor and separate pump combination. This arrangement, however, must have a means such as a universal joint or constant velocity joint to couple their shafts together. Special alignment procedures are required for this arrangement, as well as maintenance of the coupling means. Therefore, the separate motor and pump combination unit is not among those under active consideration at the present time.
Accordingly, it would be desirable to be able to detect the beginning of bearing failure within an oil pump. It would also be desirable to provide for de-energizing the motor in the event bearing wear is detected to prevent contamination by electrically conductive particles, and prevent uncontrolled contamination by non-electrically conductive particles.